Problem: Tiffany is 2 times as old as Christopher. Twelve years ago, Tiffany was 8 times as old as Christopher. How old is Tiffany now?
Solution: We can use the given information to write down two equations that describe the ages of Tiffany and Christopher. Let Tiffany's current age be $t$ and Christopher's current age be $c$ The information in the first sentence can be expressed in the following equation: $t = 2c$ Twelve years ago, Tiffany was $t - 12$ years old, and Christopher was $c - 12$ years old. The information in the second sentence can be expressed in the following equation: $t - 12 = 8(c - 12)$ Now we have two independent equations, and we can solve for our two unknowns. Because we are looking for $t$ , it might be easiest to solve our first equation for $c$ and substitute it into our second equation. Solving our first equation for $c$ , we get: $c = t / 2$ . Substituting this into our second equation, we get: $t - 12 = 8($ $(t / 2)$ $- 12)$ which combines the information about $t$ from both of our original equations. Simplifying the right side of this equation, we get: $t - 12 = 4 t - 96$ Solving for $t$ , we get: $3 t = 84$ $t = \dfrac{1}{3} \cdot 84 = 28$.